The present invention is directed to magnetic memory devices, specifically, magnetic random access memory devices. More particularly, the present invention is directed to a method and system for facilitating erasing and writing to magnetic memory devices.
Magnetic or magnetoresistive random access memory (xe2x80x9cMRAMxe2x80x9d) devices offer advantages over conventional transistor-based random access memory (xe2x80x9cRAMxe2x80x9d) devices and rewriteable nonvolatile read only memory devices. MRAM devices exploit the inherent nonvolatility of magnetic storage, used in early RAM devices, and long used in sequential memory devices in disk and tape storage. Unlike dynamic random access memory (xe2x80x9cDRAMxe2x80x9d) devices which consume appreciable quantities of power in having to be continually refreshed to preserve the integrity of their memory contents, MRAM cells do not need to be refreshed. In fact, unlike transistor-based RAM devices, once a cell of an MRAM device is polarized to its desired state, the cell retains its polarity without having to be supplied with power. Furthermore, unlike nonvolatile flash electronically erasable programmable read only (xe2x80x9cflash EEPROMxe2x80x9d) memories, the contents of which can become corrupted with heavy use, MRAM devices are highly reliable. Moreover, while flash EEPROM devices can only be rewritten by erasing them and rewriting them in their entirety, cells in MRAM devices can be selectively written and rewritten without erasing the contents stored in the entire device.
Unlike previous uses of magnetic storage, such as disk and tape storage or bubble memory, MRAM devices provide direct, random access to their contents. Accordingly, MRAM devices provide the advantages of conventional RAM devices with the reliable nonvolatility of magnetic storage.
MRAM devices exploit the inherent interrelationship between the flow of electric current and corresponding magnetic fields. As is known in the art, a current flowing through a longitudinal conductor creates a magnetic field which encircles latitudinally about the axis of the longitudinal conductor. Specifically, MRAM devices exploit this interrelationship by using electric currents to generate magnetic fields which, in turn, are applied in close proximity to storage elements comprised of magnetically susceptible materials. Electric currents directed in a first direction results in a magnetic field having a corresponding first polarity. Exposed to the field of that corresponding polarity, if the field has sufficient magnitude, the magnetically susceptible element becomes magnetized in that same polarity. The magnetic field generated by the magnetized element then is capable of reacting to other applied magnetic fields, such as those caused by other currents flowing through the conductor. As a result, if an electric current of the same polarity was applied to the same conductor which first caused the element to become magnetized, the magnetic field of that magnetized element would not resist that current. On the other hand, if an electric current of opposite polarity was applied to the conductor, inducing a magnetic field of opposite polarity to encircle the conductor, those magnetic fields would conflict, and affect the resistivity of the conductor to the flow of current. Measuring the discrepancies in current caused by the differing resistance encountered as a result of the influence of these previously magnetized elements allows the stored polarity of these elements to be read.
It will be appreciated that, while a current of opposite polarity applied to the conductor will be opposed by the current induced by the magnetic element, that current of opposite polarity will not necessarily repolarize the magnetic field of that element. Magnetic materials exhibit a hysteresis effect in that a stronger current must be applied to repolarize them than might be required to polarize them initially. This principle is relied upon by MRAM devices: currents of lower magnitude can be used to detect the magnetic field created in the magnetic elements and thereby allow the bit written to that magnetic element to be read, while currents of greater magnitude generate magnetic fields which can be used to overcome hysteresis and write or rewrite the bit written to that magnetic element. However, as is understood in the art, an acknowledged problem in MRAM devices is that relatively high currents are required to induce a magnetic field of sufficient magnitude to reliably write and rewrite MRAM memory cells.
As shown in FIG. 1, an MRAM device comprises a Cartesian array 100 of MRAM memory cells 104. Each MRAM memory cell 104 comprises an element of magnetically susceptible material 108 disposed at an intersection of a row line 112 or 116 and a column line 120, 124, or 128. Electrical current is selectively applied to the row lines 112 and 116 and column lines 120, 124, or 128 to effect writing and reading of data to and from each of the memory cells. As is known in the art, writing these cells is accomplished by selectively and simultaneously directing the current in the row lines 112 and 116 and column lines 120, 124, and 128 so as to subject a particular element 108 to a desired combination of magnetic fields generated by the current flowing through the conductive lines.
FIGS. 2A and 2B show how a magnetic element can become polarized and, therefore, written. FIG. 2A shows an MRAM cell 200 which, physically, is comprised of magnetic element 204 disposed at the intersection of the row line 208 and the column line 212. An electrical row current of a first polarity 216 is applied to the row line 208 and thereby induces a magnetic flux field 220 of a first polarity to which the magnetic element 204 is exposed. At the same time, an electrical column current 224 is applied to the column line 212 and thereby induces a magnetic field 228 to which the magnetic element 204 is exposed. The combination of these complementary magnetic fields 220 and 228 cause the magnetic element to become polarized to radiate a composite magnetic field in a predetermined direction to represent a stored data bit. Once the magnetic element 204 has become polarized, the magnetic element 204 generates a magnetic field which, as previously described, will interact with the magnetic field generated by currents of a different polarity flowing through the row line 208 and column line 212. It will be appreciated that, as in any Cartesian grid, selection of a single row and a single column singularly identify a single point on the grid. Correspondingly, applying the row current 216 to the row line 208 and the column current 224 to the column line 212 allow the individual MRAM cell 200 at the intersection of the row line 208 and the column line 212 to be programmed.
FIG. 2B, for the sake of completeness, shows the opposite case in which an MRAM cell 250 is programmed to store a magnetic field of the opposite polarity. If, in the example shown in FIG. 2A, the field stored in the magnetic element 204 of the MRAM cell 200 is considered to represent a logical zero, FIG. 2B shows the MRAM cell 250 being programmed to read as a logical one. The magnetically susceptible element 254 disposed at the intersection of the row line 258 and the column line 262 exposed to an electrical row current of a first polarity 266 applied to the row line 258 and induces a magnetic field 270 of a first polarity. At the same time, a column current 274 is applied to the column line 262 and induces a magnetic field 278 to which the magnetic element 254 is exposed. The composite magnetic field of magnetic fields 270 and 278 causes the magnetic element 254 to become polarized to radiate a magnetic fields of opposite polarity.
Once programmed, magnetic elements 204 and 254 in FIGS. 2A and 2B, respectively, will retain their magnetic fields in the absence of power. Accordingly, MRAM array 100 (FIG. 1) will retain the data stored therein where it can be retrieved upon being accessed by the system (not shown) served by the array 100 without having to be refreshed, reloaded, or rebooted.
Despite the advantages MRAM devices afford, however, they do present disadvantages. For example, because MRAM cells retain the data stored therein even when not supplied with power, affirmative steps must be taken to erase sensitive or otherwise unwanted data. One way to erase such data is to overwrite the contents of every cell in accordance with the steps described previously in connection with FIGS. 2A and 2B. Considering the hysteresis effect previously described, rewriting these cells could consume an appreciable amount of power. Writing the MRAM array with new data to be used by another application would necessarily overwrite and erase old data. However, in an age where data privacy and security becomes increasingly more important, and MRAM cells are nonvolatile, it would be desirable to be able to erase data from an MRAM array or a section thereof without having to rewrite the array with bogus data solely for the sake of erasing the data. Similarly, it would be desirable to facilitate the ability to write or rewrite MRAM cells without having to apply the high degree of current to the row and column lines required to overcome the hysteresis effect.
An additional concern arises from the possibility that the conductive row and column lines themselves could become magnetized through being exposed to the magnetic fields radiated by the magnetic elements. This could pose a problem in reading the MRAM cells. As previously described, the MRAM cells are read by applying electric currents to the row and column lines and measuring whether any resistance was encountered as a result of the magnetic fields stored in the magnetic elements at the intersections of those lines. If the conductive row and column lines were to become magnetized, thereby radiating their own magnetic fields that would affect the longitudinal flow of current through these lines, it could skew the reading of what was stored in the magnetic elements. It would be desirable to be able to demagnetize these lines.
It is to these ends that embodiments of the present invention are directed.
The present invention employs one or more switchable, close proximity electromagnets as part of the MRAM device circuit package to apply external magnetic fields to the magnetic elements and conductive lines of the MRAM array. An external magnetic field of sufficient magnitude could be induced to overwrite each of the targeted cells in the MRAM array. Alternatively, currents of decreasing magnitude and reversing polarity could be applied to the electromagnet to demagnetize the cells, as opposed to overwriting the contents of the cells.
In addition, the generated magnetic fields could be produced so as to complement the magnetic fields induced by application of current to the row and column lines of the MRAM array, thereby facilitating writing of data to magnetic elements while applying less power to the row and column lines. As described in the background of the invention, a relatively high current might be required to generate a magnetic field of sufficient magnitude to write or rewrite a memory cell. However, if an electromagnet of the present invention were used to generate an ambient magnetic field of the polarity desired to be programmed to the selected MRAM cells, a lesser current would be required. As long as the combination of the ambient field created by an electromagnet of the present invention and the localized magnetic fields created by applying appropriate row and column currents to the appropriate row and column lines, the selected MRAM cells could be written with application of currents of lesser magnitude applied to those conductive lines. In accordance with the first disclosed use of the invention, a magnetic field of sufficiently high magnitude would erase the MRAM array, while a magnetic field of lesser magnitude could be combined with the selectively applied row and column line currents to selectively write to the MRAM cells.
Further, diagonally disposed electromagnets could be used to generate these magnetic fields, and could also be used to demagnetize the conductive row and column lines. Because the conductive lines are not comprised of magnetically susceptible materials, any magnetic fields needed to demagnetize the row and column lines would not be of sufficient magnitude to affect the magnetic fields written to the magnetic elements of the MRAM cells.